The invention relates to metal wire particularly adapted for drawing to form a brass surface coating thereon, a resulting brass-coated metal wire adapted for use in wire reinforced elastomeric articles, such as vehicle tires, hoses, conveyor belts, a wire-reinforced elastomeric article containing this wire, and a method for producing the brass coated metal wire.
In various applications, such as hoses, cables, and especially tires for motor vehicles, it is known to use metal wires as reinforcement components of the article. Particularly in tires, this reinforcement is achieved by embedding coated metallic wires in the elastomeric compound material of the tire, especially when the elastomeric compound material is used in the belts and carcass of the tire. The wires have a coating to improve the adhesion of the wire to the elastomeric compound material of the tire, and to inhibit corrosion of the metal wire. The metal wire can corrode if it enters in contact with the atmosphere, for example due to damage to the elastomeric compound material, and the corrosion can spread along the wire to other portions of the tire.
Specifically, in this regard, it is known to coat steel wires with brass of the composition, in weight percent, about 70 copper and 30 zinc. The most widely used practice is to sequentially deposit coatings of copper and zinc and then heat the wire so coated for a suitable time at a proper temperature to diffuse the copper and zinc sufficiently and form the desired brass coating. In another approach the coatings are formed by co-deposition using, for example, a cyanide bath.
This prior-art practice has the disadvantage of requiring an excessive number of manufacturing steps. For example, it is necessary after the plurality of deposition steps and the diffusion step to pickle the resulting brass coating in an acid solution to remove the zinc oxide formed at the relatively high temperatures and long times necessary to achieve the required diffusion, and to insure a slight phosphorization of the wire which facilitates the subsequent drawing of the coated wire to reduce the coated wire to the desired dimensions. The diffusion step, which is typically conducted at temperatures within the range of about 450 to 500xc2x0 C., may cause a reduction in the tensile strength of the coated steel wire. The reduction can be of as much as 5% of the original tensile strength of the material. This tensile strength reduction impairs the effectiveness of the wire when used in the intended reinforcing application, particularly when used as a reinforcing component in motor vehicle tires.
It is known in the art that a strong bond between metal wires and rubber can be obtained if the metal wire is coated with a layer of brass, as described in U.S. Pat. No. 4,486,477. Various methods have been developed to form a coating on a metal wire. For example, U.S. Pat. No. 4,226,918 discloses a ferrous wire having a homogeneous coating of nickel and copper. The wire is drawn, then is thermally softened, and is pickled before entering an electrolytic bath of copper and zinc cyanide. After the homogeneous coating is deposited, the coated wire is drawn to the desired dimensions.
Similarly, U.S. Pat. No. 4,828,000 discloses a steel substrate with a brass covering layer to enhance adhesion to the rubber, wherein the covering layer has on its surface a ratio Cu/(Cu+Zn) of no more than 0.2. The reduction of copper percentage at the surface is obtained by heating the coated metal in an inert atmosphere to a temperature of between 250xc2x0 C. and 350xc2x0 C.
In accordance with the present invention, a coated metal wire is produced that provides significant advantages over prior art products of this type, particularly when employed as a reinforcing component in an elastomeric compound material of the type used in the construction of motor vehicle tires. The elastomeric compound material may be an elastomer of either natural or synthetic origin having rubber-like characteristics, comprising fillers such as carbon black and silica are added.
In the production of the brass-coated wire in accordance with the invention, was found that diffusion of deposited copper and zinc layers to form the desired alloy coating of brass results during the wire-drawing operation, after deposition of these copper and zinc coatings. Although electro-deposition is preferred for depositing these layers, other known practices including chemical vapor deposition may also be used.
It has been determined that in the vulcanization operation used in tire manufacture and during which the brass coating of the wire is adhered to the elastomeric compound material, maximum effectiveness of adhesion is obtained when the copper content of the brass coating is relatively high at the outer surface of the coating, contacting the elastomeric compound material. In fact, the outer portion of the coating that actually bonds with the elastomeric compound material, also referred to as the reaction surface, is only approximately 20 nanometers thick. The adhesive reaction between the brass and elastomeric compound material results during vulcanization by the formation of chemical bonding therebetween by disulfide bond formation. This reaction is improved and promoted by the copper of the brass alloy, because copper reacts more rapidly than zinc with the elastomeric compound material. Consequently, the bonding is facilitated by increasing the concentration of copper at the reaction surface of the brass coating. To obtain optimum adhesion, the concentration of copper at the reaction surface can be controlled according to the invention to match the characteristics of the elastomeric compound material being used, by selecting a composition and thickness of the most external deposited layer.
Further, with the relative amounts of copper and zinc in accordance with the ranges of the invention, the drawability of the coated wire is improved. Specifically, this results when the crystallographic structure of substantially all of the brass coating is the face centered cubic alpha phase with only minor amounts and preferably trace amounts of the body centered cubic beta phase. It has been determined that the face centered cubic alpha phase is significantly more deformable than the body centered cubic beta phase, and thus the predominance of this former phase facilitates the wire drawing operation.
In accordance with one aspect of the invention, a metal wire, which is particularly adapted for drawing to form a brass surface coating thereon, is provided with at least three alternate alloying layers, each being of copper or zinc. The most external of these layers is, however, of copper. Preferably, the most internal of the at least three layers is also of copper. The wire onto which these layers are deposited is preferably steel. The number of alternate alloying layers is preferably within the range of 3 to 5.
The alloying layers, in combination, may consist essentially of, in weight percent, about 60 to 72 copper and balance zinc, preferably about 70 copper and about 30 zinc.
The metal wire may have a diameter of about 0.8 to 3.0 mm with the alloying layers in combination having a thickness of about 0.75 to 4.0 microns. The thickness of the most external layer of copper is preferably about 0.1 to 0.5 microns. Preferably, the thickness of the most external copper layer could be selected to obtain a desired copper concentration on an outer surface of the coated metal wire.
The brass coating on the drawn wire preferably has a copper content of the outer surface greater than a copper content of any remaining portion of the brass coating. The brass coating preferably consists essentially of, in weight percent, about 60 to 72 copper and balance zinc, more preferably about 70 copper and about 30 zinc.
The drawn wire preferably has a diameter of about 0.12 to 0.8 mm, with the brass coating having a thickness of about 0.1 to 0.3 microns.
The brass coated wire, in accordance with the invention, may be contained as a reinforcing element within an elastomeric article, such as a motor vehicle tire. The brass coating is chemically bonded to the elastomeric article by disulfide bonds formed between the brass coating and the composition of the elastomeric article. The elastomer of the compound material may be of either natural or synthetic origin.
In accordance with the method of the invention, a brass coated metal wire is produced by depositing on a metal wire at least three alternate alloying layers each being one of copper or zinc. The most external and most internal of the layers are copper. This coated wire is then subjected to a drawing operation which produces high temperature and pressure to alloy the copper and zinc layers and form the desired brass layer. The metal of the wire is preferably steel and the number, composition and thickness of the deposited layers are as set forth above. These alloying layers may be deposited by electro-deposition.